The present invention relates generally to the field of powder metallurgy, and more particularly to articles formed from compositions of matter that include a tungsten-containing powder and at least one binder, and to methods for forming such articles.
Conventionally, many articles have been produced from lead because of lead""s relatively high density (11.3 g/cc) and relatively inexpensive cost. Examples of such articles include firearm projectiles, radiation shields and various weights. More recently, lead substitutes have been sought because of the toxicity of lead. For example, in 1996 the U.S. Fish and Wildlife Service banned the use of lead shotgun shot for hunting waterfowl. Various lead substitutes have been used, including steel and bismuth, with each offering various advantages and disadvantages as compared to lead. Other lead substitutes include tungsten or tungsten alloys.
The present invention is directed to manufacturing processes for articles that are formed compositions of matter that include powders containing tungsten and at least one binder. The manufacturing process includes compacting the mixture of powders under a first pressure to yield a desired intermediate structure, then reshaping the structure under a second pressure that is lower than the first pressure to yield the desired article. Appropriately durable tools may be used for the high-pressure compaction step, while more precise tools may be used for the lower-pressure reforming step. The composition of matter preferably is selected to reflow, or be reshaped, without fragmenting or otherwise disintegrating into discrete particles or particulate. In some embodiments, the compacted intermediate and/or final article has an extrusion constant of less than 30,000 psi. In some embodiments, the mixture of powders used to form the article have an ASTM Hall flowmeter reading for fifty grams through a cone (without tapping) of less than 18 seconds.
In some embodiments, the manufactured article contains at least one metallic binder. In some embodiments, the article contains at least one non-metallic binder, such as a polymeric binder. In some embodiments, the article contains both a metallic binder and a non-metallic binder. In some embodiments the article is a lead substitute. In some embodiments the article is a firearm projectile, such as a bullet or shot, which may be ferromagnetic or non-ferromagnetic, which may be frangible or infrangible, and which may be jacketed or unjacketed. In some embodiments, the article has a density in the range of approximately 8 g/cc and approximately 15 g/cc, with subsets of this range including densities less than the density of lead, densities selected to be equal to the density of lead or a lead alloy such as lead-antimony alloys that are commonly used in firearm projectiles, and densities selected to be greater than the density of lead, such as densities in the range of 11.5 g/cc and 15 g/cc or densities of at least 12 g/cc.